I have a mountain bike and a tourer. I'm happy with both. Now I'm looking to add a faster, lighter road bike to my stable - one I can ride around home and in centuries. I'd like something light, but I have no desire to race. I want something comfortable enough for all day in the saddle. I won't be loading it down and taking it on tour, but I'll want to carry a sandwich, a book, a multitool, and a spare tube. I don't have multiple thousands to spend, but my wife will probably let me get away with something in the $1500 range. I'm leaning towards a double crankset with a fairly wide range cassette in back. I want to go fast on flats (faster than my tourer) but still be able to get up steep hills.

Carbon fiber would probably be nice for the vibration damping and lightness, but may be too pricey. I thought maybe an aluminum frame with carbon seatstays??? I've also seen some models with curved aluminum seatstays to absorb some vibration.

I know several of the big companies offer models aimed toward someone like me.

IMHO a good steel frame is the most comfortable long distance ride going - I have a CF bike as well as a classic lugged steel bike. The steel bike climbs well and on a long ride may be faster as I stay more comfortable on it. The steel bike is about 5lbs heavier - this makes a small difference in climbing but where I recognize it more is on accelleration, the steel bike is not as responsive but I also don't have to stay on it as much to keep it at speed - mass=momentum.

However - for 1.5K you could probably find a moderately good CF package.
For 1.5K you could also probably find a good used steel frame and wrap a Centaur or Veloce group around it - thats what I did 13 years ago and have been happy ever sense. Infact I am so happy with that frame I am now pulling the Veloce off (after about 15K miles) and putting a new compact Chrous group on it.

Another candidate with similar geometry and purpose is the Salsa Casseroll (http://www.salsacycles.com/casserollComp08.html) which can be had complete for about $1500. It is also available as a frame set for the mid-$500's so with wheels and a 105 or Ultegra groupo you could bring it in totally custom equipped for around $1500 if you shopped carefully and did some of the work yourself.

Hate to say it but Fast comfortable and light does point to C.F.- unless you have more money.

Top end aluminium frames can meet all of your criteria- but they are fast being run down by the shops. C.F. is the new product but Even the stiffest aluminium frame can be improved with C.F. Forks and seat stem.

Then there is the manufacturer. Going through just one that I know-Giant- and there are several models that will fit into what you want. Trek will have the same- so will Specialised and Bianchi and Cannondale and---(just add your favourite manufacturer here).

The bike is not the problem but the dealer is. You have to find the right LBS.

Two bikes seem to stand out that 50+ers have been happy with. Specialised Roubaix in all its different forms and Giant TCR C and SCR C. And I know I have a TCR C so it is a plug for my bike- but I also have Boreas and that is my bike of choice.

Edit- Aluminium with C.F.Seat stays- Look at the Giant TCR-Alliance A1. Well worth a test ride.

I think the most comfort will come from having a bike that fits well and has bars and a saddle that agree with you. So drop a few bucks on a good fit and then start your search. One caveat at looking at fast bikes (whatever that means? race oriented maybe.) is tire clearance. A big tire will go a long way to making your bike comfortable while adding around a 100grams to your tire weight, which is not a bad trade (Schwalbe Marathon Race looks good.). To that end something like a soma smoothie es might be really nice, or even a long haul trucker from Surly. I cant think of any aluminum bikes that are road oriented that have good tire clearance, but it might be worth checking out as I have ridden some reasonably comfortable aluminum bikes and have been beat to death by some fairly stiff steel bikes. Finding a CF bike that is not a cross bike with tire clearance may be hard.